As a mobile communication system that succeeds an LTE (Long Term Evolution), standardization of an LTE-advanced in the 3GPP (Third Generation Partnership Project) has been reviewed. In the LTE-advanced, for the purpose of an improvement in a user throughput of UE (User Equipment) that is located at a cell edge, an improvement in a cell throughput, or the like, a review has been made with respect to a technology in which a plurality of transmission and reception points that are geographically distant from each other cooperate with each other and perform a MIMO (Multi-Input Multi-Output) transmission, which is called coordinated multi-point transmission and reception (CoMP), even in a downlink (DL) and an uplink (UL). The CoMP is classified into the following two technologies (A) and (B). Hereinafter, the user equipment is sometimes expressed by an UE.
(A) Joint Processing (JP)
In the joint processing, data transmitted to the UEs may be used at the plurality of CoMP transmission points, and is transmitted as a desired signal toward the UEs. In this way, large inter-cell interference (ICI) from neighbor cells is changed into a desired signal, and other-cell interference is reduced. In addition, as a sub-category of the joint processing, Joint Transmission (JT)/Dynamic Cell Selection (DCS) is defined.
(B) Coordinated Beamforming (CB) and Coordinated Scheduling (CS)
In the coordinated beamforming and the coordinated scheduling, data that is transmitted to the UEs may be used only by a serving cell, and is transmitted only from the serving cell. In addition, information such as a beamforming weight that is necessary for the coordinated scheduling and the coordinated beamforming is determined in a coordination manner among the plurality of transmission points. Therefore, the inter-cell interference may be effectively reduced or suppressed.
Both of the joint processing and the coordinated beamforming methods are effective technologies to realize high frequency usage efficiency.
As described above, in the LTE-advanced, an approach has been made with respect to a situation in which due to the coordinated MIMO transmission and reception technologies among the plurality of the transmission and reception points (for example, NodeB), inter-cell interference is reduced and therefore the high frequency usage efficiency is realized. On the other hand, in Rel. 8 description of the LTE, a configuration is also defined to realize Inter-Cell Interference Coordination (ICIC). Hereinafter, a downlink (DL) transmission from a base station to the user equipment will be described.
In the DL transmission of the LTE Rel. 8, an RNTP (Relative Narrow band TX Power), which indicates information of future transmission signal power (normalized EPRE (Energy Per Resource Element)) in physical resource block (PRB) units, is defined as a binary indicator (0 or 1) (refer to Non-Patent Literature 3).
Here, 1 PRB is made up by 12 resource elements (RE), that is, 12 sub-carriers. Due to an inter-cell coordinated scheduling in which an indicator of future transmission signal power (RNTP) in other cells is used, the ICIC in a frequency domain may be realized. Hereinafter, the physical resource block is sometimes expressed simply by a PRB.
FIG. 22(A) shows a conceptual diagram of a case in which the indicator of the RNTP is shared between cells. In addition, FIG. 22(B) shows the indicator of the RNTP for each PRB in a frequency domain.
As shown in FIG. 22(A), NodeB #0 (serving cell) notifies NodeB #1 (non-serving cell) of the indicator of the RNTP via a backhaul (for example, X2 interface). In addition, the NodeB #1 performs the coordinated scheduling of the frequency domain by using the indicator of the RNTP of the NodeB #0.
In addition, as shown in FIG. 22(B), the indicator of the RNTP is information of future transmission signal power for each PRB. In a case where a normalized EPRE (transmission signal power density) is smaller than any RNTP threshold value (that is, the transmission signal power is reduced in the future), the indicator of the RNTP shows “0”. In addition, the upper limit of the normalized EPRE is not promised (no promise), the indicator of the RNTP shows “1”.
That is, the NodeB #0 notifies the NodeB #1 of information of the transmission signal power for each PRB, for example, via the backhaul such as X2, and the NodeB that has received the information does not allocate the UE of a serving cell to the PRB showing the indicator of 1 and may perform the coordinated scheduling of the frequency domain, in which interference with neighbor cells is reduced. That is, the NodeB #0 may effectively realize an FFR. (Fractional Frequency Reuse) by considering other-cell interference.
However, in the transmission system of LTE Rel. 8, an effect of reducing interference with other cells and a decrease in the frequency usage efficiency through the sharing of a band between cells are in a trade-off relationship, such that there is a problem in that an improvement effect of the throughput is not sufficient. Here, the “sharing of a band between cells” is equivalent to a case in which frequency repetition (reuse factor) is partially set to be more than 1 in the system band.
Therefore, as described above, in the LTE-advanced, a review has been made with respect to the inter-cell interference coordination (ICIC) on the same time-frequency, which uses a spatial domain (MIMO of a plurality of transmission and reception points that are geographically distant from each other). As a method of controlling interference between coordination cells by using the spatial domain, the following method has been reviewed (for example, refer to Non-Patent Literature 1 and Non-Patent Literature 2).
In a Single-User (SU)-MIMO in a single cell or a Multi-User (MU)-MIMO in the related art, when selecting a precoding matrix indicator (PMI), which is defined for precoding (beamforming) of a transmission signal, only a precoding (beamforming) in a serving cell to which user equipment of a serving cell is connected is taken into consideration. In addition, a method in which the user equipment selects a PMI that is capable of maximizing the throughput (that is, Best PMI) and feeds back the PMI to a serving cell has been adopted. On the other hand, in Non-Patent Literature 1 and Non-Patent Literature 2, on the basis of extended feedback information related to the PMI (beamforming weight) from the UE in which interference from other cells is taken into consideration, the PMI information (for example, a PMI restricting the use in neighbor cells, that is, worst PMI) is shared between coordination nodes. In addition, on the basis of the shared information, the coordinated beamforming (CB) is performed among the plurality of cells, and thereby the maximization of the throughput in the entirety of the plurality of cells is attempted. Hereinafter, specific means thereof will be illustrated with reference to FIG. 23. FIG. 23 shows a conceptual diagram in a case where the coordinated beamforming (CB) among the plurality of cells is performed on the basis of the PMI information.
(1) The UE receives a reference signal (RS) of other cells (and a serving cell) and determines a worst PMI list of other cells that generate large inter-cell interference (ICI) with itself. In addition, the UE feeds back the worst. PMI list (worst PMI #0 and worst PMI #3 in the drawing) to the serving cell (refer to arrow (1) in FIG. 23).
(2) The NodeB #0 that is a serving-cell selects the worst PMI list on the basis of feedback information from a plurality of UEs that are connected to the serving cell, and notifies other nodes of the selected worst PMI list (refer to arrow (2)-2 in FIG. 23).
(3) Other nodes select a PMI other than the notified worst PMI and transmit it to the UE in the serving cell (connected to the node) in a precoding (beamforming) manner (refer to (3) in FIG. 23).
In this manner, interference with neighbor cells may be reduced while avoiding the decrease in frequency usage efficiency by sharing the band between the coordination cells (the number of frequency repetitions (reuse factor) is partially set to be more than 1 in the system band), which is a problem in the transmission system of LTE Rel. 8.